The present invention relates to an improved plasma torch method and apparatus, and, more particularly, to an improved method and apparatus for plasma spraying.
An important use of the direct current (DC) plasma torch is in applying spray-coatings to surfaces. A coating material, in powder or wire form, is fed into a heat source created by an electric arc which passes through a gas and ionizes the gas to form a plasma. The intense heat from the plasma arc melts the coating material into small droplets, hereinafter referred to as "spray particles," and projects them at high velocity onto a prepared target substrate for coating. The resulting spray-coating is hard and resistant to mechanical abrasion as well as to the effects of high temperatures.
As illustrated in FIG. 1, to which reference is now made, a basic prior art plasma generator consists essentially of a hollow cylindrical anode 14 and a concentric inner cathode 10 located near one end of anode 14 and insulated from anode 14 by an insulator 12. A power supply 30 has the negative output terminal thereof connected to cathode 10 and the positive output terminal thereof connected to anode 14. Power supply 30 supplies sufficient current at a sufficient voltage to create and sustain a plasma arc between cathode 10 and anode 14. Cathode 10 is sometimes referred to as the "back electrode." The gas which is to be ionized is forced under pressure into the cavity 18 of anode 14 through a hole 16 in cathode 10, and flows toward an exit nozzle 20 located at the front end of anode 14. A high-current DC arc is initiated from cathode 10 to anode 14, and it is this plasma arc which creates the plasma in cavity 18. The gas which is to be ionized is usually introduced into cavity 18 in a tangential direction when viewed in a transverse cross-section, and the plasma arc which is created and sustained therein assumes a swirl form having an angular rotational component of velocity, this swirl form herein referred to as a "plasma vortex." The present application uses the term "direction of the plasma vortex" to denote the direction of the flow of ionized gas in the plasma vortex. By adjusting the flow of gas down the axis of anode 14, it is possible to force the plasma arc to attach to anode 14 at different distances from cathode 10. The plasma arc in this configuration of plasma generator tends to attach to anode 14 at a small point, and this mode of plasma arc attachment is referred to as the "contraction attachment mode." The contraction attachment mode is characterized by a very high current density, approximately 10.sup.9 amperes per square meter. It is this high current density which causes rapid erosion of the anode. A coating material is introduced through a hole 22 in exit nozzle 20, where the coating material is melted into spray particles and accelerated toward the target substrate as shown. During operation, outside cavity 18 there is a region 24 of flame-like high enthalpy, high temperature, and high velocity. The present application uses the term "plasma generator" to denote any device for plasma spraying having a hollow cavity, the inner wall of which acts as one electrode for an electric arc, and through which an ionized gas can flow.
There are a number of shortcomings and deficiencies of current plasma generators, which include the following. First the erosion rate of the electrodes is excessive, on the surface 26 of cathode 10, and particularly on anode 14 in the region of the inner wall 28. Second, the enthalpy of the plasma is below the desired level. Third, there is inadequate control over the chemical composition of the spray-coating and the dimensions of the spray particles, their adhesion to the target substrate, and their gas permeability. Fourth, the plasma velocity has an angular rotational component which, for a given kinetic energy, reduces the axial component of the plasma exit velocity, and therefore reduces the ability of the plasma to accelerate the spray particles. Fifth, the cost of gases such as Argon, Helium, and Nitrogen for use in plasma generators is high. In addition, it is desirable to increase the bond strength, microhardness, corrosion resistance, and deposition rate of the spray-coatings over the levels currently attained, and it is also desirable to decrease the porosity and the oxide content of the spray-coatings below the levels currently attained.
Various attempts have been made to address these issues. For example, direct current (DC) plasma generators are utilized in spite of the fact that they involve higher cost and complexity than alternating current (AC) plasma generators, because AC plasma arcs have higher contamination and instability levels, lower enthalpy, and higher nozzle erosion rates. U.S. Pat. No. 2,920,952 to Ducati, et al. discloses methods and apparatus for increasing the useful life of the back electrode. In "Linear Direct Current Plasma Torches" (page 9-43 of Thermal Plasma and New Materials Technology, volume 1), M. F. Zhukov summarizes a number of advances in plasma generator design, including segmented anode inserts, gas-swirling interelectrode inserts, and rotating magnetic fields. The purpose of these innovations is to lengthen, control, and stabilize the plasma arc, in order to increase the enthalpy of the plasma and to reduce the electrode erosion. As another example, Israel patent number 103069 to B. Goodman, which is incorporated by reference as if fully set forth herein, discloses a current modulation technique and a method of using a fuel and oxidizer mixture to produce an explosive detonation in the plasma, to increase the acceleration of the spray particles.
FIG. 2 schematically illustrates one of the above-mentioned advances, that of the segmented anode plasma generator. This configuration has a cathode 40 and a segmented anode made of separate annular anode segments 42, 44, and 46. At the exit point of the segmented anode is an exit nozzle 48, after which is a coating material feed 52, where the coating material is introduced and melted into spray particles. One of the advantages of the segmented anode plasma generator is that the attachment point of the plasma arc will be on the inside surface 50 of anode segment 46, rather than at some other point, and so the length of the plasma arc can easily be maintained at a desired level. This is because the positive output terminal of power supply 30 is connected only to segment 46. Segment 42 and segment 44 are electrically floating, and therefore a plasma arc cannot exist between cathode 40 and segment 42, or between cathode 40 and segment 44. In contrast, the configuration shown in FIG. 1 has a plasma arc which can establish the attachment thereof to any point on the inside surface 28 of anode 14. Consequently, the configuration shown in FIG. 1 has what is referred to as a "self-establishing plasma arc". Such a plasma arc can have an arc length which is less than optimum. The enthalpy of the plasma decreases as the arc length thereof decreases, and it is therefore desirable to maintain as long a plasma arc as possible, and the segmented anode plasma generator thereby represents an improvement over the earlier design.
Despite these advances, however, existing plasma generators still fall short of the potential which can be achieved. There is thus a widely recognized need for, and it would be highly advantageous to have, methods and apparatus which would further decrease the rate of electrode erosion, further increase the enthalpy of the plasma, further increase the axial component of the plasma velocity, further increase the velocity of the spray particles, and make other improvements in order to increase the tensile bond strength, microhardness, corrosion resistance, and deposition rate of the spray-coatings, to decrease the porosity and the oxide content of the spray-coatings, and to enable the application of amorphous and ceramic thermal barrier coatings. These goals are met by the present invention.